1. Field of the Invention
The present invention relates to a multibeam exposure head which exposes a recording material such as a photosensitive material, a photosensitive material or a heat-sensitive material, forming an image using multiple beams, and to a multibeam exposure apparatus using the multibeam exposure head.
2. Description of the Related Art
Conventionally, in the technical field of printing, litho-plate-making using a presensitized plate (PS plate) has widely been practiced. For example, litho-plate-making for multicolor printing is performed as described below. A color image is read using a scanner by decomposing its colors into three colors: red (R), green (G) and blue (B). Image signals representing the three colors are converted into color-decomposed halftone-dot-signals for four colors: cyan (C), magenta (M), yellow (Y) and black (Bk). Exposure printing on a photosensitive material called lith film is performed using a light beam modulated on the basis of the color-decomposed halftone-dot-signal for each color to obtain a lith plate for the corresponding color. Exposure printing of a halftone dot image in each color on a PS plate is performed using the corresponding lith plate, thus making printing plates for four colors C, M, Y and Bk for litho printing.
In recent years, however, direct plate-making and computer to plate (CTP) technology has been popular because of their advantage of simplifying the printing process and reducing the plate-making time. In direct plate-making or CTP, to make a printing plate for each of four colors C, M, Y and Bk, direct drawing on a PS plate with a light beam such as a laser is performed using the corresponding color-decomposed halftone dot signal obtained by a scanner system without making and using lith films in intermediate steps.
On the other hand, the recording density needs to be increased to 2400 dpi, 3600 dpi and 5000 dpi for increasing halftone levels and image quality of printed images. The plate-making time needs to be shortened while increasing the recording density to such a level. There is a demand for high-density drawings in a shorter time not only in the printing field but also in other various image recording fields.
However, an apparatus cannot be realized, which is capable of performing such high-density drawing with one light beam because the number of revolutions of a drum around which the PS plate needs to be fitted and which is rotated for scanning in the main scanning direction must be set to 10000 r.p.m. or greater. This can be established, considering any structural, control and manufacturing-cost conditions. By considering this problem, multibeam exposure apparatuses have been proposed in which simultaneous exposure recording for several lines is performed using one row of light beams.
Any of such multibeam exposure apparatuses use an optical fiber array or the like in the form of a row of optical fibers. The direction of one row of fibers in the optical fiber array is tilted from a main scanning direction to reduce a pitch of multiple beams emitted from the optical fiber array according to a selected resolution, thereby enabling exposure recording on a PS plate to be performed while changing resolution between various values, e.g., 2400 dpi, 3600 dpi and 5000 dpi. If an optical fiber array having a larger number of optical fibers arranged in a row is used to effectively reduce the exposure recording time at once, it is necessary to increase the number of optical fibers arranged in a row. If the number of optical fibers arranged in a row is increased, the width of arrangement of multiple beams from the optical fiber array is necessarily increased since the lower limit of pitch of the optical fibers is set depending on the fiber diameter. Further, it is necessary to correspondingly increase a size of optical system lenses for imaging with the multiple beams on the PS plate. Therefore necessity for increasing the size of the exposure apparatus arises as well as the need for using low cost performance optical system lenses, resulting in increasing manufacturing cost of the exposure apparatus.
An optical fiber array of a dual-row-structure having two rows of optical fibers arranged parallel to each other may be provided to set a certain number of beams larger than that in the single-row structure without larger optical system lenses. In an optical fiber array of such a dual-row structure, however, even if the direction of arrangement of the lower row of optical fibers, for example, is tilted from the main scanning direction to obtain the desired resolution in the same manner as that in the above-described conventional arrangement, the upper row of optical fibers does not suitably cooperate with the lower row of optical fibers, so that it is difficult to set the pitch for the desired resolution.
Further, when an optical fiber array of a dual-row structure is used, the imaging magnification may be reduced to 1/1.5 by optical system lenses to change e.g. resolution from 2400 dpi to 3600 dpi. However, because the original imaging magnification is ordinarily 0.5 or less, e.g., about 0.33, the focal depth of multiple beams immediately before exposure on the PS plate is shallow. Further, when the imaging magnification is reduced, the focal depth becomes much shallower. In such a situation, when the position of the surface of the rotating drum around which the PS plate is fitted and which is rotated for main scanning, is changed due to a small eccentricity of the drum, for example, eccentricity of about 10 xcexcm, beam spots formed by the multiple beams are defocused by this change. The beam spots of the multiple beams are also defocused owing to a small curvature in the optical system lenses. Therefore, an excessive reduction in imaging magnification caused by optical system lenses must be avoided and it is practically impossible to reduce the imaging magnification by 50% or more for a change of resolution by using optical system lenses.
In view of the above-mentioned problems, it is an object of the present invention to provide a multibeam exposure head and a multibeam exposure apparatus arranged to perform image exposure recording at a desired resolution with substantially no change in imaging magnification of the optical system by using an optical fiber array of a dual-row-structure.
In order to attain the above object, the following aspects will be provided by the preset invention.
The first aspect of the present invention is characterized by a multibeam exposure head having a multibeam light source which exposes a recording material by main scanning, the multibeam light source having a first multiple beam forming light source in which a plurality of beam emitting ports are arranged parallel to each other while being spaced apart from each other by a predetermined distance, and a second multiple beam forming light source in which a plurality of beam emitting ports are arranged parallel to each other being spaced from each other by the predetermined distance, the plurality of beam emitting ports in the second multiple beam forming light source being placed parallel to the parallel arrangement direction of the beam emitting ports in the first multiple beam forming light source while being spaced apart by a predetermined distance from the same, and the position of the beam emitting port at one end of the second multiple beam forming light source being shifted in the parallel direction relative to the position of the beam emitting port at the corresponding end of the first multiple beam forming light source.
Further it is preferable that the head further has a tilt angle changing unit which makes, by rotating the multibeam light source, a change in exposure condition from a first exposure condition in which each of first multiple beams emitted from the first multiple beam forming light source and each of second multiple beams emitted from the second multiple beam forming light source are alternatively arranged at an equal interval in a subscanning direction perpendicular to the direction of main scanning on the recording material, to a second exposure condition in which each of the first multiple beams and each of the second multiple beams are alternatively arranged at an equal interval in a subscanning direction.
Further it is preferable that the head further has an optical system in an optical path between the multibeam light source and the recording material, from a first beam pitch formed on the recording material through the optical system by each of the first multiple beams and the second multiple beams alternatively arranged at equal intervals in the subscanning direction under the first exposure condition, the multibeam light source being rotated by using the tilt angle changing unit to form a desired second beam pitch on the recording material through the imaging optical system by each of the first multiple beams and the second multiple beams alternatively arranged at equal intervals in the subscanning direction under the second exposure condition.
Further it is preferable that the head in a case where the arrangement distance of the beam emitting ports is Df; the first beam pitch is P; the second beam pitch is Q; and imaging magnification of the optical system is M, and in a case where a distance by which the first multiple beam forming light source and the second multiple beam forming light source are spaced apart from each other by a predetermined distance is Wf, then Wf obtained by the following equation (1) is set:
Wf=Lxc2x7cos(xcex8a+"PHgr"1)/Mxe2x80x83xe2x80x83(1)
where L=(((2xc2x7nxe2x88x921)xc2x7Q+Pxc2x7cos(xcex94xcex8))/sin (xcex94xcex8))2+P2)1/2,
xcex8a=cosxe2x88x921(2xc2x7P/(Dfxc2x7M)),
"PHgr"1=sinxe2x88x921(P/((((2xc2x7nxe2x88x921)xc2x7Q+Pxc2x7cos(xcex94xcex8))/sin(xcex94xcex8))2+P2)1/2),
xcex94xcex8=cosxe2x88x921(2xc2x7Q/(Dfxc2x7M))=cosxe2x88x921(2xc2x7P/(Dfxc2x7M)), and
n is a natural number.
Further it is preferable that the head in a case where a width by which the position of the beam emitting port of the second multiple beam forming light source is shifted in the parallel arrangement direction relative to the position of the beam emitting port of the first multiple beam forming light source is Af, then Af obtained by the following equation (2) is set:
Af=(Wfxc2x7Mxc2x7sin(xcex8a)+P)/(cos(xcex8a)xc2x7M)xe2x80x83xe2x80x83(2)
Further it is preferable that the head of the optical system have a lens which finely adjusts imaging magnification of the optical system, the lens being provided in an optical path of the first multiple beams and the second multiple beams.
Further it is preferable that the head includes the multibeam light source having an optical fiber array.
The second aspect of the present invention is characterized by a multibeam apparatus having the multibeam exposure head described above according to the first aspect and an outer drum capable of performing main scanning on the recording material by having the recording material fitted and rotated around its outer cylindrical surface.